A solar ceil which is eco-friendly alternative energy is an apparatus which uses electrons and holes generated by absorbed photons to convert light energy into electric energy. In detail, the solar cell has a p-n junction structure in which a positive (p) type semiconductor is functioned to a negative (N) type semiconductor and thus when receiving sunlight, the solar cell generates holes and electrons within the semiconductors due to energy of the incident sunlight and generates a potential as the holes (+) move to the p type semiconductor and the electrons (−) move to the n type semiconductor by an electric field generated at the p-n junction, such that the solar cell produces power.
The solar cell may be classified into a substrate type solar cell and a thin film type solar cell. The substrate type solar cell uses a semiconductor material such as silicon as a substrate and mainly uses a bulk type crystalline silicon substrate. The solar cell may have high efficiency and stability, but may be expensive, may be difficult to make a thickness of an absorption layer thin, and may perform an intermittent process. Meanwhile, the thin film solar cell is fabricated of amorphous silicon, thin film polycrystalline silicon, copper indium gallium diselenide (CIGS), cadmium telluride compound (CdTe), organic materials, and the like and therefore may make the thickness of the absorption layer thin and may use glass, metal, or plastic as a substrate and therefore may be continuously mass-produced to be economical.
The thin film solar cell is configured of a substrate, a lower electrode which is formed on the substrate, an absorption layer which absorbs light to generate electricity, a window layer through which light passes, and a superstrate for protecting the lower layers. In this case, the absorption layer uses a p-type semiconductor and the window layer uses an n-type semiconductor to have a p-n diode structure.
As a material forming the light absorption layer, the thin film type solar cell uses CuInSe2 which is a base, and may use CuGaSe2 using gallium (Ga) instead of indium (In) or a quaternary material of CU(In, Ga)Se2 simultaneously using indium (In) and gallium (Ga). Further, the thin film type solar cell may use CuInS2, Cu(In, Ga)S2, or the like in which selenium (Se) is substituted into sulfur (S) and a rive component-based material of Cu(In, Ga) (Se, S)2 simultaneously using selenium (Se) and sulfur (S).
A band gap is controlled by adding other elements to CuInSe2, thereby increasing light-electricity conversion efficiency. In this case, when the thin film type solar ceil has the same composition in a thickness direction of the absorption layer, a the thin film type solar ceil has a predetermined band gap, but forms grading in the thickness direction of the thin film due to the added element and thus makes carrier collection easy due to the formed electric field, thereby increasing the light-electricity conversion efficiency. In particular, as compared with a single grading method which constantly increases the band gap in the thickness direction, a double grading method which controls the band gap in a V-letter type may more increase efficiency by 2 to 3%, such that the implementation of the double grading method is essential to a high-efficiency solar cell.
In the thin film type solar cell, the light absorption layer is fabricated by using co-evaporating metal elements or binary compounds as a main material or co-sputtering an alloy of Cu, In, and Ga, depositing these elements on the substrate, and then selenizing these elements. In this case, a method of fabricating an absorption layer using the co-evaporating grows the (In, Ga) Se layer into a crystal at a temperature of about 350° C. and increases the temperature of 350° C. to a high temperature of 550 to 600° C. and then deposits a second CuSe layer. The CIGS is simultaneously formed by reaction between the previously deposited IGS layer and a newly deposited CS layer. When the CS reacts with the IGS, a reaction rate of Cu—In is more rapid than that of Cu—Ga and therefore Ga has a higher concentration (grading) toward a lower electrode layer and when the first IGS is entirely converted into the GIGS, a third IGS layer is deposited. The Cu concentration is in a Cu rich GIGS state higher than stoichiometric CIGS immediately before the third IGS layer is deposited, and the IGS layer is converted into Cu deficient GIGS while being additionally deposited. Further, similar to the first layer, when the third layer is deposited, Cu is diffused to the third IGS layer which is being deposited, in which Ga has a higher concentration toward a buffer layer and a window layer which are to be deposited later and thus the double grading method may be implemented. However, as the double grading uses a high temperature of about 550 to 600° C., large-area uniformity may not be secured due to deflection, and the like in the case of using general soda-lime glass and therefore it is difficult to implement a large area and material utilization is low and therefore production cost may be increased.
Meanwhile, in the case of using a sputtering method as a method of depositing a light absorption layer, after Cu—Ga and In are sputtered, a process of performing selenization or sulfuration on the sputtered Cu—Ga and In is used, but forms a void within, the absorption layer at the time of selenization, and the like, such that it is difficult to fabricate a solar cell having high efficiency and reliability.